TMCI Observations in AD Summary of 1999 Observations F. Pedersen, CERN/AB/RF July 2006 TMCI Observations AD

Outline Modes of AD operation during 1998 – 2000 commisioning Commissioning: original and revised plans AD ring and beam parameters Instability threshold and observations Can studies be repeated? July 2006 TMCI Observations AD

AD operation modes, Commissioning 1998 - 2000 Protons via loop, counter clock wise rotation, normal magnet polarity N = 2 – 5x109 protons, optics set-up, high beam density required for deceleration to 100 MeV/c (T = 5 MeV) Failed due to TMCI instabilities Protons direct via target area, clock wise rotation, reversed magnet polarity N = 5x109 protons, initial stochastic cooling setup Normal antiproton operation, clockwise rotation, normal magnet polarity N = 2 – 5x107 antiprotons Difficult beam diagnostics due to low intensity July 2006 TMCI Observations AD

AD Modes: pbar mode Normal operation, low intensity pbars: 2 – 4x107 Difficult beam diagnostics due to low intensity (tunes, intensities, orbits..) Each phase plane (EH, EV, EL) compressed 5000 [EH, EV] to 25000 [EL] (six-dimensional phase space density increased 6x1011 !!!) With improved beam diagnostics (tunes, intensity, Dp/p, orbits), most setting up was done in this mode from 2000 onwards Access to experimental hall OK July 2006 TMCI Observations AD

AD Modes: protons direct Used for initial setting up of stochastic cooling and electron cooling at 300 MeV/c Intensity typically 2x109 (limited by stochastic cooling speed) DC beam transformer works (marginally) Magnetic fields does not re-produce when inverted (ex. coupling) Large blow-up (EH, EV, EL), due to 40 m air path in ‘dog-leg No access to hall July 2006 TMCI Observations AD

AD Modes: protons via loop No cooling possible with CCW rotation: very dense injected beam required Intensity typically 3-5x109 (limit: transverse mode coupling instabilities) Tune measurements at low energy impossible due to large Dp/p required to control TMCI, unable to decelerate to 100 MeV/c (Dp/p = 0.7% at 0.3 eVs) Difficult to correct and maintain small injection oscillations Used for set-up of ‘dog-leg’ transport No access to experimental hall July 2006 TMCI Observations AD

Commissioning Original and revised plans 6 months (1998/99) scheduled for commissioning Set up optics, orbits, deceleration and tunes with protons via loop Initial cooling set-up in protons direct mode, reverse polarity Final cooling set-up with antiprotons, use protons via loop as necessary to measure orbits and tunes After 1½ years we were still unable to decelerate protons from loop to 100 MeV/c due to either transverse (TMCI) or longitudinal blow-up (needed to combat TMCI) In 1999/2000, a major RF shielding effort of RF and BPM systems made it possible to measure tunes (BTF) and orbits with 2 – 4x107 pbars Commissioning completed successfully in July 2000 (design emittances or better, 4 times design intensity) using pbars only. Protons via loop used last time in 2003. 3.5 GeV/c Loop and one injection kicker module decommissioned since then… July 2006 TMCI Observations AD

AD Ring and Beam parameters Circumference, frev 185 m, 1.589 MHz Momentum range 3.57 –> 0.1 GeV/c Gamma transition gtr 4.8 Slip factor, h 0.021, ginj = 3.94 (3.57 GeV/c) Longitudinal Z/n ~1600 ohms, K-S crit., 12 SC tanks [AC] RF Voltage VRF 3 kV, h = 1 Synchr. freq. fs 86 Hz @ 3 kV, ns = 5.4x10-5 Intensity, Nprotons 3 – 5x109, longit. and transv. shaving in PSB AD Acceptance, AH, AV 180 pi mm.mrad Emittances, EH, EV 0.4 – 0.6 pi mm.mrad [95%, in PS] Expected emittances 3.5 pi mm.mrad [95%, AD, Al 0.1 mm windows] Longit emittance EL 0.12 –> 0.3 eVs [controlled blow-up in PS] Measured AD emittances 2.5 – 3.5 pi mm.mrad [95%, below TMCI threshold] Measured AD emittances 7 - 9 pi mm.mrad [95%, above TMCI threshold] July 2006 TMCI Observations AD

Instability Thresholds, Observations We monitor injection oscillations (H&V) by observing a low frequency betatron line (6 - Qx) x frev and minimize with vertical steering dipoles, septum and kicker We look for signs of coherent instabilities by observing a fixed high frequency betatron line (39 - Qx) x frev near the bandwidth limit of the pick-up system: ~61 MHz With large longitudinal emittance (>0.3 eVs), small enough injection oscillations, and intensities below 4x109, the expected transverse emittances can be achieved: 2.5 – 3.5 pi mm.mrad With small longitudinal emittance (0.12 eVs) and small injection oscillations we observe much large emittances than expected (7 - 9 pi mm.mrad): Noisy coherent signals (filamentation following instability) appears at high frequency betatron lines 100 – 200 msec after injection From profile measurements we observe that the central density in transverse phase space is depleted relative to larger amplitude, Filamentation signals and transverse emittance blow-up disappear when the longitudinal emittance is increased (PS longitudinal blow-up) July 2006 TMCI Observations AD

Instability Thresholds, Observations No beam loss results from the instability. Most likely tune spread from octupolar fields stabilize the blow-up before the aperture limit (~180 pi mm.mrad) is reached If the aperture is limited artificially with scrapers to a value slightly larger than the expected emittance, a large fraction of the beam is lost on a time scale in the order of a synchrotron period (10 ms): the instability is amazingly fast considering the low intensity – not classical head-tail modes. The AD is a ‘dirty’ machine in terms of coupling impedances: 4 stochastic cooling tanks with guts optimized to maximize the transfer impedance (12 tanks in the AC era), Z/n ~1.6 k ohms estimated from longitudinal coasting beam instabilities The large transverse coupling impedance and the low synchrotron frequency (close to transition, low RF voltage and harmonic number) results in a low TMCI threshold: From Chao and Tigner p. 136 (B. Zotter) July 2006 TMCI Observations AD

Vertical beam profile, above instability threshold Graph shows losses (scintillator counts) versus scraper position Central phase space density only half of density a few mm’s from the centre when instability is present Could also be result of poor injection steering, but injection oscillations were carefully monitored July 2006 TMCI Observations AD

Horizontal beam profile, below instability threshold Measured emittance close to expected value: 2 pi mm.mrad High phase space density in center of beam July 2006 TMCI Observations AD

Could TMCI studies be repeated? The objective in 1999 was not to study the TMCI in the AD, but to provide some useful test beam (protons or antiprotons) at 100 MeV/c The last operation with protons via loop took place in 2003 (beam intensity calibration of RF and Schottky pick-up) Since then, the 3.57 GeV/c loop has been decommissioned, ADTST beam is no longer in regular operation, and one of the injection kicker modules has been removed from the machine Protons direct (and using AD reversed polarity) suffer a large emittance blow-up (~80 – 100 pi mm.mrad) from the transport in air over ~40 meters, and the instability will most likely not be observed With pbars the intensity is 100 times lower and the longitudinal emittance 16 times larger.. It would be difficult and very expensive to repeat these studies… July 2006 TMCI Observations AD

Source of Information AD Logbook III 15 July 1999 -> December 1999 (paper log) Injection oscillation studies, 21 October 1999, afternoon shift TMCI studies, 6 November 1999, morning shift Good transverse emittances injected: 8 November 1999, afternoon shift In 2000, ‘protons via loop’ mode was abandoned in favor of normal antiproton mode thanks to improvements in low intensity beam diagnostics…. July 2006 TMCI Observations AD